1. Field of the Invention
The present invention relates to a steering apparatus for a vehicle, which is used for steering the vehicle in accordance with steering operations performed on steering means by a driver.
2. Description of Related Art
A vehicle is steered by transmitting steering operations performed on steering means by a driver in the passenger's room to a steering mechanism disposed outside the passenger's room so as to turn tire wheels for steering (right and left front wheels, in general) by movement of the steering mechanism.
Recently proposed as one of steering apparatuses for achieving such steering in Japanese Patent Application Laid-Open No. 10-218000 (1998) by the present applicant is a steering apparatus of separate type, or of what is called steer-by-wire type, wherein steering means in the passenger's room is provided mechanically separately from a steering mechanism at the outside of the passenger's room. In the steering apparatus of separate type, an actuator for steering, which is attached to a part of the steering mechanism, is moved on the basis of detection results of an operation direction and an operation amount of the steering means, realizing steering according to steering operations performed on the steering means.
The steering apparatus of separate type has advantages that correspondence between the steering amount of the steering means and the movement amount of the actuator, or what is called a steering transfer ratio, can be set freely without mechanical limit and that steering characteristics can be controlled and changed flexibly in response to a running state such as a vehicle speed, a swing speed and existence of change in speed. It should be noted that an electric motor is widely used as the actuator on account of the easiness to control and change the steering characteristics.
The aforementioned steering apparatus of separate type has advantages that no limit exists in the structure and arrangement of the steering means and that suitable steering means to be used can be selected. However, many of the conventionally proposed steering apparatuses of separate type, including the steering apparatus disclosed in Japanese Patent Application Laid-Open No. 10-218000, employ steering means (which is hereinafter referred to as steering-wheel-shaped steering means) formed in imitation of a steering wheel which is generally used in a steering apparatus of non-separate type.
In a steering apparatus of non-separate type, a ring-shaped steering wheel is attached to one end of a column shaft mechanically connected with a steering mechanism and a turn of the column shaft according to a turning operation of the steering wheel is mechanically transmitted to the steering mechanism, realizing steering by movement of the steering mechanism according to the transmission. It should be noted that a steering apparatus configured as a power steering apparatus provided with an actuator aiming at reduction of driver's load is in practical use, the actuator being driven to apply steer-assisting force to a steering mechanism on the basis of a detection result of the steering torque which is applied to the column shaft by steering operations performed on the steering wheel.
The steering-wheel-shaped steering means employed in the steering apparatus of separate type, which is provided with a ring member fixed on one end of a turning shaft mechanically separated from the steering mechanism, detects the turning direction and turning amount of the turning shaft due to the turning operation of the ring member and moves the actuator for steering attached to the steering mechanism on the basis of the detection result. It should be noted that a reaction force motor is attached to the middle portion of the turning shaft as reaction force applying means. Turning force of the reaction force motor is transmitted to the turning shaft via reduction means to perform the turning operation of the ring member against the turning force, allowing the driver to feel reaction force against steering which is practically applied to the steering mechanism.
The steering apparatus of separate type provided with the aforementioned steering means has an advantage of providing the same feeling in steering as the steering apparatus of non-separate type, however, has a problem that the load put on the driver by repeated steering is large and occurrence of tiredness after driving for a long time is unavoidable since steering of such steering means requires a large operation amount. Furthermore, there arises another problem that a physically handicapped driver meets difficulties in steering of such a steering apparatus.